I. Field of the Invention
The present invention relates generally to linear actuators and, more particularly, to an electric motor driven linear actuator.
II. Description of Relevant Prior Art
There are many previously known linear actuators that are powered with electric motors. These previously known linear actuators typically comprise a housing having a plunger slidably mounted to the housing and movable between an extended and a retracted position. An electric motor is drivingly connected to the plunger to move the plunger between its extended and its retracted position in response to activation of the motor.
In many situations, it is desirable to move the linear actuator between preselected extended and retracted positions. In order to selectively deactivate the motor whenever the plunger reaches its retracted or extended position, these previously known linear actuators have typically employed limit switches which are activated by the plunger whenever the plunger is in its extended or retracted position.
These previously known linear actuators with limit switches, however, have not proven entirely satisfactory in use. One disadvantage of these previously known linear actuators with limit switches is that they oftentimes fail after extended and repeated use. When this occurs, the limit switch no longer deactivates the motor when the plunger reaches either its extended or retracted position in the desired fashion.
A still further disadvantage of these previously known linear actuators with limit switches is that the limit switches increase the overall cost of the linear actuator.
A still further disadvantage of these previously known linear actuators is that it is oftentimes desirable upon system initialization to determine if the linear actuator was in its extended position, its retracted position or a transition position in between the extended and retracted positions at system startup. For example, in the automotive industry such linear actuators are used, inter alia, to control the shift between a four-wheel and two-wheel drive transmission of an automotive vehicle. In the event that the linear actuator was in its retracted position or extended position at system startup, it would be normally desirable to leave the linear actuator in that position. Consequently, if an automotive vehicle was in the four-wheel transmission mode at engine startup, it would normally be desirable to simply leave the vehicle in the four-wheel transmission mode, or vice versa.
Conversely, if the linear actuator was in a state of transition between its extended and retracted positions upon engine startup or system initialization, it would be desirable to actuate the linear actuator to move to a preselected one of its extended and retracted positions on system initialization rather than leave the linear actuator in a transition position. The previously known linear actuators, however, fail to account for the condition where the linear actuator was in an intermediate position at the time of the last system shutdown and thus at the time of the next system initialization. Unless corrected upon system initialization, the partially actuated linear actuator may create unpredictable and undesirable results.